As known in this technical field, overcurrent and overvoltage on a communication line of a momentary or continuous nature are caused by lightning or by contact of the line by a high-voltage cable as a result of damage from a natural disaster such as an earthquake, storm or flood; and it is necessary to prevent damage to communication equipment or possible loss of life which may result therefrom. Prior art safety connectors are known which include a discharge tube and an overcurrent protective element, and overcurrent and overvoltage on the inner line of the communication equipment is prevented by grounding any overcurrent or overvoltage caused by lightning or contact with a high-voltage line.
The prior art discharge tubes comprises ceramic discharge tubes and include two electrode tubes (diodes) and three electrode tubes (triodes). The two electrode tube is disposed leading line and grounding protection of a set of communication line, respectively, and heat is produced at high temperature when overvoltage is continuously held, but because the two electrode tube (diode), separated respectively, is covered with outer cylindrical tube and lower melting lead (Pb) connecting with the leading line is melted and interrupted by production of heat of the device, production of heat for the housing of the safety device is hardly in existence. However, when the overvoltage is led, only discharge tube of a led line is operated and the communication equipment is protected. Accordingly, on the near other line which overvoltage is not led, the overvoltage is induced and the line is affected adversely and effective protection for the communication equipment against overvoltage is not provided.
The triode has a set of communication lines and is a relatively simple structure. Although overvoltage is led on one line discharge, which is produced by ionization of gas trapped within the discharge tube, is advanced and grounding is made to other line. Accordingly, the problems of a diode are overcome, but in the structure of the triode, the problem of heat could not be overcome. Specifically when in discharge, high temperature heating results and the triode is not practically used because the safety connector and housing have anxiety for production of fire (High temperature: that is, when AC of 5A is conducted on the triode discharge tube, heat at about 100.degree. C. is produced after 1 second (Sec), 250.degree. C. after 2 Sec., 500.degree. C. after 3 Sec., 600.degree. C. after 4 Sec., 650.degree. C. after 5 Sec., and up to about 1000.degree. C. after 10 Sec.). The specification of the ceramic discharge tube, which has been used by the Korean Telecommunication Authority (100, Sejongro, Chongro-Gu, Seoul, The Republic of Korea), is described as follows: After AC of 5A is conducted for 1 second the AC is interrupted for 3 minutes. The AC is then again conducted for 1 second and interrupted for 3 minutes--such processes are repeated and tested 5 to 10 times. Under the above conditions, the discharge tube should be maintained self-functioning. As other conditions, the housing should not be modified or fire not produced by production of heat on the discharge tube when AC of 5A is applied for 15 minutes.
The present inventor has filed a prior patent application in order to meet the above-listed conditions and to overcome the above-described problems. In the prior application a ceramic discharge tube is provided in which discharge thereof is stopped and the production of fire is prevented by mounting bimetals between electrodes of the discharge tube and the ground. The bimetals expand when the temperature of the discharge tube increases, and make contact with the ground, resulting in about zero volts in the potential difference between electrodes. The prior patent application by the present inventor, however, has many problems to be solved. Specifically, when the overvoltage is led by thermal expansion properties of the bimetal, the discharge tube produces heat. The operating time in which contact by thermal expansion of the bimetal is accomplished becomes a variable parameter, dependent on minor differences of the materials of the bimetals and on the mounting space of the bimetals.
Further, in order to accomplish fast contact of the bimetal, the spacing between the bimetal and the ground should be maintained within a very narrow range (about 0.1-0.3 mm). However, in this case, the risk of fire is increased because heat of about 600.degree. C. to 700.degree. C. is produced by the continuous discharge for about 5 seconds until operating time, and the insulation properties are reduced by dust and moisture due to the narrow spacing.
Also, when constant spacings must be maintained, the mass production of products having constant operating properties is very difficult to achieve. In particular, the spacing of the bimetal is easily changed by contact with other articles during the production process.
Further, the rate of corrosion is high because the bimetal has an iron component (Fe), and the resistance of contact on the bimetal also becomes high and the accuracy of the operation according to the standard specification can not be expected when the flow of current is great.
Korean Utility Model Reg. No. 11754 (Inventors: KI HO CHUNG et al, "Safety Connector for Communication: filed Nov. 29, 1973 under Korean UM Application No. 6577/1973) discloses a "Safety Connector for Communication" in which the current leading portion of contacts is lowered because heat of the heat coil is produced and low melting temperature lead (Pb) fixing "Notice" indicating lamp is melted, and at same time, "Notice" indicating rod is shown by raisingly projecting. The safety connector is thus intended not to be again used when the unit comprising the heat coil and "notice" indicating lamp are once operated, and a new safety connector must be substituted. Furthermore, structure of this safety device is very complex and uneconomical, and, in addition, the communication equipment can not be operated until the substitute safety connector is installed.
Also in the prior art, the heat coil is wound on a bimetal connected to the outer line and the fixed contact is placed on the corresponding part with a moving contact on the bimetal. If overcurrent is produced, heating of the heat coil results and the bimetal is bent (or curved) and the moving contact of the bimetal makes contact with the fixed contact of ground terminal side and the overcurrent is grounded.
In the above-described prior art structures, if the overcurrent is led it is grounded, and if the factor producing the overcurrent is obviated, leading of current on the communication equipment side is automatically accomplished. Thus, the above-described problems are solved, but current intermittence on the communication equipment side cannot be operated speedily and precisely because the bimetal must be bent by heating of the heat coil such that the moving contact makes contact with the fixed contact of the ground terminal side when the overcurrent is led.
Further, as described on the specification of U.S. Pat. No. 4,692,833 (Inventor: KI HO CHUNG, issued Sept. 8, 1987), a safety device for communication is provided which includes a triode. The triode has a difference in operating time of expansion and contact of the bimetal by the material of the bimetal and the gap between the electrodes, and when the gap between both electrodes and the bimetal adhered to the ground electrode is maintained within the range of from about 0.1 mm to 0.3 mm in order to achieve relatively rapid contact operation, an operating time of 5 seconds is still necessary. Accordingly, in this case, high heat of about 600.degree. C-700.degree. C. is produced by the continuous discharge and there is danger of fire.